Magnetic nanoparticles (MNPs) are emerging as promising candidates for their applications in biomedical research encompassing of drug delivery, magnetic resonance imaging, cell mechanics, hyperthermia, in vivo tracking of stem cells, tumor progression, nucleic acid (DNA and RNA) separation and cell separations, due to their ultra fine sizes, biocompatibility and superparamagnetic behaviour. Another important property which makes magnetic nanoparticles ideal for biomedical applications is their low toxicity. The MNPs can have high level of accumulation in the target tissues or organ due to their host cell tropism and biophysical nature, which helps for the most promising application of these magnetic nanoparticles in site-specific drug delivery.
For drug delivery, the magnetic nanoparticles are required to have high magnetization values, size smaller than 100 nm and narrow distributions of particle size. To these nanoparticles, a pharmaceutical drug can be loaded on to the surface which could be driven to the target organ and released there. An external localized magnetic field gradient may be applied to a chosen site to attract drug-loaded magnetic nanoparticles from blood circulation, by reducing their systemic distribution and offering a possibility of administering lower but more accurately targeted dose. In this process, the magnetic nanoparticles should bear superparamagnetic property i.e, they do not retain any magnetic property when the magnetic filed is removed.
Drug targeting to tumors and its other related pathological conditions, is desirable since anticancer agents demonstrate nonspecific toxicities that significantly limit their therapeutic potentials. For these applications, the size, charge and surface chemistry of the magnetic nanoparticles are particularly important, which strongly affects both the blood circulation time as well as the bioavailability of the particles within the body. It is envisioned that nanoparticles can be surface-modified so that it could to simultaneously function as contrast enhancement agent and drug carrier, allowing real-time monitoring of tumor response to drug treatment.
Surface coating is an integral part of all MNP formulations meant for biomedical applications. The colloidal electro stabilization arising from repulsion of the surface charge are not sufficient enough to prevent aggregation in the biological solution due to presence of salts and other electrolytes that may neutralize the charge. Furthermore, on intravenous injection the MNP is subjected to the adsorption of plasma protein or opsonization as a first step of clearance by the reticuloendothelial system (RES). Accordingly evading the uptake by RES and maintaining a long plasma half life is a major challenge for many MNP applications in drug delivery. So, a polymeric coating over the MNPs is required for providing steric barrier and to prevent nanoparticle agglomeration, thereby avoiding opsonization. Also these coatings provide a way to functionalize the surface of MNPs such as surface charge and chemical functionalization. Therefore, to improve their biocompatibility and injectibility magnetic nanoparticles are generally coated with hydrophilic polymers such as starch or dextran, polyethyleneglycol (PEG), streptavidin, poly-L-lysine (PLL), poly ethylene imide (PEI), and the therapeutic agents of interest which are chemically conjugated or conically bound to the outer layer of polymer. This approach is complex, involves multiple steps with a very little drug loading capacity, and the bound drug dissociates within hours. Fast release of drug from the carrier system may be less effective, especially in the tumor therapy, where drug retention is required for therapeutic efficacy. Entrapping the magnetic nanoparticles into other sustained release polymeric drug carrier systems such as nanoparticles formulated from poly-dl-lactide-co-glycolide (PLGA), polylactides (PLL), polylactic acid (PLA), or in dendrimers results in significant loss in magnetization of the core magnetic material. Also in silica coated magnetic nanoparticles there is decrease in magnetization which has the limitation for the effective targeting in drug delivery system.
Various monomeric species such as bisphosphonates, dimercaptosuccinic acid and aloxysilane have been evaluated to facilitate the anchoring and attachment of polymers on MNP. But coating of the particle with monomeric species does not allow colloidal stability at physiological pH. Coating the particles with large molecules, such as polymers or surfactants containing long-chain hydrocarbons, helps to prevent aggregation of the particles in biological solution thereby offering more effective stabilization. Therefore, different research groups mostly use long chain polymer such as oleic acid and its salt for the stabilization of iron oxide nanoparticles. Gupta et al have synthesized magnetic nanoparticles by coprecipitation method using sodium oleate for forming stable dispersion of magnetic nanoparticles. Jain et al have developed oleic acid (OA)-pluronic (F-127) stabilized iron oxide magnetic nanoparticle formulation where they have entrapped some of the hydrophobic drugs which partitioned into it without any loss of magnetization. As in their study they found that after a coating of OA, still these formulations were not well dispersible in water, so they have used pluronic types of surfactants to get water based formulation. The pluronic acid anchors at the interface of the OA shell and give the aqueous dispersibility and easy load of hydrophobic anticancer agents. Experimental evidences show that higher doses of pluronic (F-127) have toxic effects towards human erythrocytes and there is an elevation of cholesterol and triglycerides in the blood plasma.
Therefore, with an aim of getting colloidal stability of the magnetic nanoparticles without use of any surfactant, a different polymeric lipid molecule was used for coating of the MNPs. Synthetic lipid glyceryl monooleate (GMO) approved by food and drug administration (FDA), is an emulsifier, flavouring agent for the food industry and excipient agent for antibiotics. The ionic polymer GMO also possesses bioadhesive properties that can be used to enhance the therapeutic efficacy of the dosage forms by increasing the contact time at the targeted tissues. Glyceryl monooleate (GMO) is an unsaturated monoglyceride belonging to the class of water-insoluble amphiphilic lipids. Depending on the water content and temperature it forms different types of lyotropic liquid crystals. As water content and temperature increase, it system forms cubic phase via reversed micellar and lamellar phases. The cubic liquid crystalline phase is highly viscous, thermodynamically stable, and insensitive to salts and solvents and coexists in equilibrium with excess of water and resistant to physical degradation. The high viscosity of GMO provides sustained release of drugs due to slow drug diffusion or increased residence time in its solubilized form. The heterogeneous structure of GMO in water permits incorporation of both hydrophilic and hydrophobic drugs or a combination and their presence does not induce a change in lyotropic phase structure. GMO is a metabolite during lipolysis of triglycerides. Also, GMO itself is an object of lipolysis due to different kinds of esterase activity. Hence, the cubic phase made of GMO is biodegradable and, as such a potential candidate for use in drug delivery systems. GMO has a similar long chain polymer structure as that of oleic acid, mainly used in the formulation of MNPs. Keeping in view of these properties of GMO; we have coated the magnetic nanoparticles with GMO by replacing OA. We have developed a novel aqueous based ultrafine stable magnetic nanoparticle formulation with a coating of GMO without the use of any surfactant. The aqueous solubility of the particle is achieved by the complete removal of the un-adsorbed GMO during the washing process with the use of different organic solvents during the synthesis process. We hypothesize that, GMO coated MNP will be a ideal delivery system for the treatment of cancer as the hydrophobic drug would partition into the GMO coating and would provide aqueous dispersibility of the solutions without any loss of magnetization and at the same time drug loaded MNPs can be used as a novel drug delivery system with the help of external magnetic field.
Bioseparation
MNPs are beneficial in biomedical research for separating out the specific biological entities from their native environment in order to concentrate the samples for further analysis. It is possible due to attraction between an external magnetic field and the MNPs which enables the separation of a wide variety of biological entities. Use of biocompatible MNPs is one of the ways to achieve this. It is a two step process involving i) tagging or labeling of the desired biological entities with magnetic material and ii) separating out these tagged entities via fluid based magnetic separation devices. Labeling is achieved through the surface modification of magnetic nanoparticles with dextran, phospholipids and Polyvinyl alcohol (PVA) which provides the link between the particles and the target site on a cell or molecules. To aim for specific binding on the surface of the cells, the help of antibody and antigen specificity action can be taken into account. For active binding the cells are targeted with biological molecules such as hormones and folic acid. Precision binding of antibodies specifically to their corresponding antigens provides an accurate way to label cells e.g MNPs coated with immunospecific agents have been successfully bound to red blood cells HIV-tat peptides, lung cancer, bacteria, urological cancer cells and golgi vesicles. The magnetic separation of target cells from mixtures, such as peripheral blood, isolation of cancer cells in blood samples or stem cells in bone marrow has considerable practical potential in improved diagnosis in biomedical research. When combined with microfluidic technology, low-field magnetic separations could enable faster and less expensive processing of tissue samples for biomarker detection. Furthermore, MNPs can be biologically activated to allow the uptake of cells via endocytic pathways, thereby allowing certain cellular compartments to be specifically addressed. Once taken up, the desired cellular compartments can be magnetically isolated and accurately studied using proteomic analysis. There are two main challenges to make all the above-discussed biomedical applications come true: a) a good synthesis route for manufacturing monodisperse MNPs with diameters <10 nm; and b) a good method to functionalize the surface of the nanoparticles. The latter determines the ability of the MNPs to interact in a well-defined and controllable manner with living cells and to be used for the cell separation. We have functionalized acid groups on the surface of the GMO coated magnetic nanoparticles by the use of DMSA (2, 3 meso mercapto succinic acid) which can be further conjugated with the primary amine groups of any peptide or protein etc.